1. Field of the Invention
This invention generally relates to serial communications and, more particularly, to a system and method for mapping asynchronous tributary streams into a high data rate Optical Transport Network signal without buffering or the use of phase-locked loops (PLLs).
2. Description of the Related Art
Conventional Optical Transport Network (OTN) architecture (standardized in ITU-T G.872, G.709) permits the transport of several lower order/lower bit-rate Optical channel Data Unit of level j (ODUj) over a higher-order/higher bit-rate Optical channel Data Unit of level k, ODUk (k>j). The multiple lower bit-rate ODUj tributaries are Time Division Multiplexed (TDM) into the higher bit-rate ODUk (k>j). Currently defined are ODU1 (about 2.7 Gbit/s), ODU2 (about 10.7 Gbit/s), ODU3 (about 43 Gbit/s), and the ITU is working on the definition of ODU4 (about 112 Gbit/s).
FIG. 1 is a diagram of the OTUk frame structure (prior art). OTN multiplexing is a layered structure including an Optical Payload Unit of level k (OPUk) defined within Optical Data Unit of level k (ODUk), which is defined within Optical Transport Unit of level k (OTUk). OPUk is the information structure used to adapt client information for transport over an optical channel. It includes client information together with any overhead needed to perform rate adaptation between the client signal rate and the OPUk payload rate, and other OPUk overhead needed to support the client signal transport. OPUk capacities for k=1, k=2, and k=3 are defined. Although not yet defined, k=4 will keep the same frame structure.
The ODUk is an information structure consisting of the information payload (OPUk) and ODUk related overhead. ODUk capacities for k=1, k=2, and k=3 are defined. The optical channel transport units of level k (OTUk and OTUkV) are the information structure used for transport of ODUk over one or more optical channel connections. OTUk is a completely standardized optical transmission unit of level k and the OTUkV is a functionally standardized optical transmission unit of level k. Each consists of the optical data unit and OTUk related overhead, such as forward error correction (FEC) and overhead for management of an optical channel connection. OTUk is characterized by its frame structure, bit rate, and bandwidth. OTUk capacities for k=1, k=2, are k=3 are currently defined.
An OPUk frame includes a number of Tributary Slots (TS), which are interleaved within the OPUk. A Tributary Slot includes a part of the OPUk overhead (OH) area and a part of the OPUk payload area. The Optical channel Data Tributary Unit j into k (ODTUjk) is an information structure that carries a justified ODUj signal. The bytes of the ODTUjk frame are mapped into the OPUk payload area of the Tributary Slot. The bytes of ODTUjk Justification Overhead are mapped into the OPUk OH area.
The ODUj tributary signals may be carried by different network clocks, thus, they are asynchronous to each other. In particular, for ODU2 signals, rates different from the standard rate are adopted in the network due to non-standard implementation. As the transmission networks begin migrating to higher rates at over 40 Gbps (OTU3) and 100 Gbps (OTU4), the transport of lower bit-rate, i.e. ODU2, signals into higher bit-rate OTU3 or OTU4 signals becomes costly if conventional mapping mechanisms are used, as each one of the 4 or 10 ODU2 signals requires an independent PLL to provide clocking for the transport of the tributaries over to a higher bit-rate OTU3 or OTU4 muxing device.
With the asynchronous mapping of ODU2 signals into ODTU23/ODTU24, the justified signals are synchronized into a common clock. The ratio between an OTUk signal rate and the ODTUjk rate requires an m/n scaling of OTUk signals to provide a clock rate that is common to both ODTUjk and all the ODUj tributaries.
It would be advantageous if ODUj tributaries could be mapped into an ODTUjk frame without multiple PLLs. It would be advantageous if each ODUj tributary could be mapped into the ODTUjk frame using a single shared clock.